Highly active, robust and versatile multifunctional, fully non-noble metals based electro-catalyst compositions and methods of making for energy conversion and storage

ABSTRACT

The invention provides noble metal-free electro-catalyst compositions for use in acidic media, e.g., acidic electrolyte. The noble metal-free electro-catalyst compositions include non-noble metal absent of noble metal. The non-noble metal is non-noble metal oxide, and typically in the form of any configuration of a solid or hollow nano-material, e.g., nano-particles, a nanocrystalline thin film, nanorods, nanoshells, nanoflakes, nanotubes, nanoplates, nanospheres and nanowhiskers or combinations of myriad nanoscale architecture embodiments. Optionally, the noble metal-free electro-catalyst compositions include dopant, such as, but not limited to halogen. Acidic media includes oxygen reduction reaction (ORR) in proton exchange membrane (PEM) fuel cells, and direct methanol fuel cells and oxygen evolution reaction (OER) in PEM-based water electrolysis and metal air batteries, and hydrogen generation from solar energy and electricity-driven water splitting.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. § 119(e) from U.S.provisional patent application No. 62/265,503, entitled “HIGHLY ACTIVE,ROBUST AND VERSATILE MULTIFUNCTIONAL, FULLY NON-NOBLE METALS BASEDELECTRO-CATALYST COMPOSITIONS AND METHODS OF MAKING FOR ENERGYCONVERSION AND STORAGE”, filed on Dec. 10, 2015, the contents of whichare incorporated herein by reference.

1. FIELD OF THE INVENTION

This invention relates to non-precious, non-noble metal-basedelectro-catalyst compositions for use in an acidic media, e.g., acidicelectrolyte conditions, methods of preparing these electro-catalystcompositions and, forming composites and electrodes therefrom. Moreparticularly, the electro-catalyst compositions exclude the presence ofany noble precious metals and are useful for proton exchangemembrane-based water electrolysis, proton exchange membrane fuel cells,direct methanol fuel cells and metal-air batteries.

2. BACKGROUND

The performance and efficiency of energy generation and storagetechnologies are dependent on the nature of the electro-catalyst. Ingeneral, high performance electro-catalysts are utilized in fuel cells,hydrogen generation from solar (i.e., photocatalytic andphotoelectrochemical) and electrolytic water splitting and metal airbatteries. Noble metals-based electro-catalysts are known in the art toexhibit excellent electrochemical performance, such as, lowoverpotential and superior reaction kinetics, as well as long-termstability, in acidic media, e.g., acidic electrolyte, for oxygenreduction reaction (ORR) in proton exchange membrane (PEM) fuel cellsand direct methanol fuel cells (DMFCs), oxygen evolution reaction (OER)in PEM-based water electrolysis and metal air batteries, and hydrogengeneration from solar energy and electricity driven water splitting.

PEM based fuel cells (PEMFCs, DMFCs) and PEM based water electrolyzershave several advantages over alkaline/neutral based systems. Theadvantages include higher energy efficiency, superior production rates,increased product purity and more compact design. During PEM fuel celloperation, ORR ensues at the cathode and hydrogen oxidation occurs atthe anode of the fuel cell. In summary, water and electrical current isproduced. In a PEM-based water electrolyzer, the current flow and theelectrodes are reversed and water decomposition takes place. Oxygenevolution occurs at the anode (abbreviated “OER”, i.e., oxygen evolutionreaction) and reduction of protons (H⁺), which travel through themembrane, takes place at the cathode. As a result, water is decomposedinto hydrogen and oxygen by means of current. The high capital costs ofcurrent electrolyzers is due to one or more of the following: deploymentof expensive noble metal-based electro-catalysts (e.g., IrO₂, RuO₂, Ptand the like), use of relatively small and comparatively low efficiencysystems, customized power electronics, and labor intensive fabrication.Similarly, the use of expensive and precious noble metals-basedelectro-catalysts (e.g. Pt, Pd) contributes to the high capital cost ofPEM based fuel cells (PEMFCs, DMFCs).

Rutile-type noble metal oxides, such as IrO₂ and RuO₂, are well knownanode electro-catalysts for OER in alkaline and PEM-based waterelectrolysis. However, the anodic over-potential and the cell resistancein electrolysis contribute to a majority of the losses observed inelectro-catalytic performance. In addition, IrO₂ and RuO₂electro-catalysts undergo electrochemical or mechanical degradationunder extreme and highly corrosive electrochemical environmentsprevalent in acid-assisted water electrolysis which reduces theperformance with time and diminishes the service life of the electrodeduring OER.

The development of PEM-based systems has been slowed by the use andavailability of only expensive noble metal-based electro-catalysts, suchas Pt, IrO₂ and the like. Thus, there are advantages to reducing theamount, or even precluding the presence of noble metal inelectro-catalysts. Likewise, there are concerns that such reduction orpreclusion of noble metal will result in low or poor electrochemicalperformance. These concerns are particularly relevant forelectro-catalysts that are employed in harsh acidic conditions, such as,an acidic electrolyte, which is associated with PEM-based systems.

The amount of noble metal present in an electro-catalyst can be reducedby combining the noble metal with a non-noble metal component. Forexample, it has been known to reduce the amount of the noble metalcomponent by mixing transition metals and/or transition metal compounds,such as, oxides, with noble metals including Pt for ORR and noble metaloxides, such as, IrO₂ and/or RuO₂ for OER. Further, it is known thatcombining a higher percentage of non-noble metals and a lower percentageof noble metals can contribute to producing an electro-catalystexhibiting favorable properties. However, even though the cost of suchelectro-catalysts is less than a pure noble metal-basedelectro-catalyst, they are still expensive and can be cost prohibitivein certain applications.

Alternatively, the noble metal present in an electro-catalyst can becompletely replaced by a non-noble metal component. Several non-noblemetal-based electro-catalysts have been identified for use in alkaline-and neutral-based water electrolysis and fuel cells. For example,Mn-oxide (e.g., MnO₂ and Mn₃O₄), spinel (e.g., NiCo₂O₄) and La-basedoxide electro-catalysts (e.g., LaNiO₃ and LaCoO₃) have been used inalkaline/neutral fuel cells and water electrolysis. However, there areseveral disadvantages associated with these materials. In particular, ithas been difficult to design and develop non-noble metal-basedelectro-catalysts that exhibit at least comparable performance andpreferably, superior performance, and stability as compared to noblemetal- and noble metal oxide-based electro-catalysts. Further, MnO_(x)was found to exhibit poor stability in acidic media, as well as lowelectronic conductivity, which was not favorable for fast chargetransfer during electro-catalytic process. Thus, MnO_(x)-basedelectro-catalysts were not considered suitable for use in PEM fuelcells.

Thus, there is a desire and need in the art to design and developelectro-catalysts that are composed of non-noble metals in the absenceof any precious, noble metals and, capable of exhibiting one or more ofsuperior electronic conductivity, excellent charge transfer kinetics,high electrochemical active surface area, outstanding electrochemicalactivity for OER and ORR, superior long-term electrochemical stabilityand excellent methanol tolerance for use in direct methanol fuel cellcathodes.

SUMMARY OF THE INVENTION

In one aspect, the invention provides a noble metal-freeelectro-catalyst composition for use in acidic media that includes anon-noble metal selected from the group consisting of manganese oxide,copper oxide, zinc oxide, scandium oxide, cobalt oxide, iron oxide,tantalum oxide, tin oxide, niobium oxide, tungsten oxide, titaniumoxide, vanadium oxide, chromium oxide, nickel oxide, molybdenum oxide,yttrium oxide, lanthanum oxide, neodymium oxide, erbium oxide,gadolinium oxide, ytterbium oxide, cerium oxide and mixtures thereof.The noble metal-free electro-catalyst is absent of noble metal.

The noble metal-free electro-catalyst composition can also include adopant selected from the group consisting of at least one element fromGroup III, V, VI and VII of the Periodic Table. In certain embodiments,the dopant can be selected from the group consisting of fluorine,chlorine, bromine, iodine, sulfur, selenium, tellurium, nitrogen,phosphorus, arsenic, antimony, bismuth, aluminum, boron and mixturesthereof. The dopant can also be present in an amount from greater than 0to about 20 weight percent based on the total weight of the composition.In preferred embodiments, the dopant can be present in an amount fromabout 10 to about 15 weight percent based on the total weight of thecomposition.

The noble metal-free electro-catalyst composition can have a generalformula I:

(a _(x) b _(y))O_(z) :wc  (I)

wherein, a is Mn, Cu, Zn, Sc, Fe, Co, Ti, V, Ni, Cr, Ta, Sn, Nb, W, Mo,Y, La, Ce, Nd, Er, Gd, Yb or mixtures thereof b is Mn; O is oxygen; c isF, Cl, Br, I, S, Se, Te, N, P, As, Sb, Bi, Al, B or mixtures thereof; xand y are each a number greater than or equal to 0 and less than orequal to 2, x and y may be the same or different, when x=0, y is anumber greater than 0 and less than or equal to 2 and when y=0, x is anumber greater than 0 and less than or equal to 2; z is a number that isgreater than 0 and less than or equal to 4 (for all x and y), and w isfrom 0% by weight to about 20% by weight, based on the total weight ofthe composition.

In certain embodiments, the noble metal-free electro-catalystcomposition can include manganese and copper oxide, and have a formulaof Cu_(1.5)Mn_(1.5)O₄. The dopant can be fluorine, which can be presentin an amount of 0, 5, 10 or 15 percent by weight of the totalcomposition, such that the formula is Cu_(1.5)Mn_(1.5)O₄,Cu_(1.5)Mn_(1.5)O₄:5F, Cu_(1.5)Mn_(1.5)O₄:10F, orCu_(1.5)Mn_(1.5)O₄:15F, respectively. In other embodiments, theelectro-catalyst composition can include manganese and copper oxide, andhave a formula of Cu_(1.5)Mn_(1.5)O_(2.75):1.25F.

The form of the non-noble metal-based electro-catalyst composition caninclude, but is not limited to, nano-particles, a nanocrystalline thinfilm, nanorods, nanoshells, nanoflakes, nanotubes, nanoplates,nanospheres, nanowhiskers and combinations thereof.

The noble metal-free electro-catalyst composition can also be at leastpartially coated on a current collector substrate.

The acidic media can be selected from oxygen reduction reaction in aproton exchange membrane fuel cell and oxygen evolution reaction in aproton exchange membrane based water electrolysis.

In another aspect, the invention provides a method for preparing a noblemetal-free electro-catalyst composition for an acidic electrolyte. Themethod includes combining a non-noble metal oxide precursor with aprecipitation agent or reaction agent to form a non-noble metal oxideprecipitate, separating the precipitate, drying the precipitate, andforming a noble metal-free oxide powder.

The method can further include introducing a dopant precursor prior tothe step of drying the precipitate, to form a solid solution.

The non-noble metal oxide precursor can be a salt, such as manganeseacetate tetrahydrate, the precipitation agent or reaction agent can bepotassium permanganate, the precipitate can be manganese dioxidenanoparticles, the dopant precursor can be a fluorine-containing salt,such as ammonium fluoride, and the powder can be fluorine-dopedmanganese oxide powder.

In still another aspect, the invention provides a method for preparing anoble metal-free electro-catalyst composition for an electrode. Themethod includes preparing a non-noble metal oxide nanomaterial, adding ahalogen precursor, forming a precipitate, dispersing the precipitate insolvent to form a sol gel, and drying the sol gel to form a non-noblemetal oxide powder.

BRIEF DESCRIPTION OF THE DRAWINGS

A further understanding of the invention can be gained from thefollowing description of the preferred embodiments when read inconjunction with the accompanying drawings in which,

FIG. 1a is a plot that shows the XRD patterns of amorphous MnO₂ andCu_(1.5)Mn_(1.5)O₄:F of varying fluorine contents;

FIG. 1b is a TEM image of Cu_(1.5)Mn_(1.5)O₄:10F indicating the uniform(˜8-10 nm) nanosized range of the catalyst particles generated;

FIG. 1c shows the ¹⁹F MAS NMR spectra of Cu_(1.5)Mn_(1.5)O₄:5F,Cu_(1.5)Mn_(1.5)O₄:10F, Cu_(1.5)Mn_(1.5)O₄:15F andCu_(1.5)Mn_(1.5)O₄:20F indicating the presence of fluorine (spinningside bands are marked by asterisks);

FIGS. 2a and 2b are plots that show the iR_(Ω) corrected polarizationcurves for OER of Cu_(1.5)Mn_(1.5)O₄:F of varying fluorine content(total loading of 1 mg/cm²) and IrO₂ (total loading of 0.15 mg/cm²)obtained in 0.5 M H₂SO₄ solution at 40° C.;

FIG. 2c is a bar graph that shows a comparison of the electrochemicalactivity for OER of Cu_(1.5)Mn_(1.5)O₄:F of varying fluorine content(total loading of 1 mg/cm²) and IrO₂ (total loading of 0.15 mg/cm²) at1.55 V (vs RHE);

FIG. 2d is a plot that shows the cyclic voltammograms ofCu_(1.5)Mn_(1.5)O₄:F of varying fluorine content (total loading of 50μg/cm²) and Pt/C (Pt loading of 30 μg_(Pt)/cm²), in N₂-saturated 0.5 MH₂SO₄ at 26° C.;

FIG. 2e is a plot (magnified view) that shows the cyclic voltammogramsof Cu_(1.5)Mn_(1.5)O₄:F of varying fluorine content (total loading of 50μg/cm²) and Pt/C (Pt loading of 30 μg_(Pt)/cm²), in N₂-saturated 0.5 MH₂SO₄ at 26° C.;

FIG. 3a is a plot that shows the iR_(Ω) corrected polarization curvesfor ORR of Cu_(1.5)Mn_(1.5)O₄:F of varying fluorine content (totalloading of 50 μg/cm²) and Pt/C (Pt loading of 30 μg_(Pt)/cm²) inO₂-saturated 0.5 M H₂SO₄ solution at 26° C. with rotation speed of 2500rpm;

FIG. 3b is a bar graph that shows a comparison of electrochemicalactivity for ORR of Cu_(1.5)Mn_(1.5)O₄:F of varying fluorine content(total loading of 50 μg/cm²) and Pt/C (Pt loading of 30 μg_(Pt)/cm²) at0.9 V (vs RHE);

FIG. 3c is a plot that shows the variation of current density vs timefor OER of Cu_(1.5)Mn_(1.5)O₄:10F (total loading=1 mg/cm²) and IrO₂(total loading=0.15 mg/cm²) performed in 0.5 M H₂SO₄ solution under aconstant potential of 1.55 V (vs RHE) at 40° C. for 24 hours; and

FIG. 3d is a plot that shows the iR_(Ω) corrected polarization curve(initial and after 6000 cycles) for ORR of Cu_(1.5)Mn_(1.5)O₄:10F (totalloading=50 μg/cm²) obtained after stability test, obtained inO₂-saturated 0.5 M H₂SO₄ solution at 26° C. with rotation speed of 2500rpm and scan rate of 5 mV/sec, indicating outstanding electrochemicalstability with negligible loss in electrochemical activity for ORR forlong term operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to non-precious, non-noble metal-basedelectro-catalyst compositions absent of noble metal for use in acidicmedia, e.g., acidic electrolyte conditions. The invention provides lowcost, highly active, durable, electro-catalysts completely free orabsent of precious, noble metals for oxygen reduction reaction (ORR) inproton exchange membrane (PEM) fuel cells and direct methanol fuelcells, and oxygen evolution reaction (OER) in PEM based waterelectrolysis and metal air batteries.

In order to achieve high electrochemical performance, it is typical foran electro-catalyst to be composed of noble metal. However, due to thehigh cost associated with the high performance noble and preciousmetals, it is advantageous to reduce the amount of noble metal in theelectro-catalyst compositions. It is even more advantageous to precludethe use of noble metal completely. Although, a disadvantage of the useof an electro-catalyst that is devoid of noble metal is the expectedlyreduced, or even poor, electrochemical performance. Thus, as previouslydescribed, there have been found in the prior art various combinationsof noble metal and non-noble metal electro-catalyst compositions.Further, there has also been found non-noble metal electro-catalystcompositions for use in alkaline or neutral media. It has provendifficult to date thus far, however, to prepare a completely non-noblemetal based electro-catalyst for acidic media, e.g., acidic electrolytethat performs comparable to noble metal electro-catalyst compositions.The present invention provides completely non-noble metal-basedelectro-catalyst compositions absent of noble metal for use in acidicmedia that demonstrate one or more of superior electro-catalyticactivity, excellent electrochemical stability and moreover, lowover-potential that is typically exhibited by noble metalelectro-catalyst compositions.

The electro-catalyst compositions according to the invention aresuitable for use in a variety of applications that employ harsh acidicconditions, such as, an acidic electrolyte, for ORR in PEM fuel cellsand direct methanol fuel cells, and OER in PEM-based water electrolysisand metal-air batteries.

The electro-catalyst compositions of the present invention include oneor more non-noble metals. Suitable non-noble metals for use in thisinvention include a wide variety that are known in the art, such as,non-noble metal oxides, such as, but not limited to, manganese oxide,copper oxide, zinc oxide, scandium oxide, cobalt oxide, iron oxide,titanium oxide, nickel oxide, chromium oxide, vanadium oxide, tantalumoxide, tin oxide, niobium oxide, tungsten oxide, molybdenum oxide,yttrium oxide, lanthanum oxide, neodymium oxide, erbium oxide,gadolinium oxide, ytterbium oxide, cerium oxide and, combinations andmixtures thereof.

The form of the non-noble metal oxide can include any configuration of asolid or hollow nano-material. Non-limiting examples of suitablenano-materials include myriad configurations and geometries of nanoscalearchitectures, such as, but not limited to, nano-particles, ananocrystalline thin film, nanorods, nanoshells, nanoflakes, nanotubes,nanoplates, nanospheres, nanowhiskers and combinations thereof, ofmyriad nanoscale architecture embodiments.

It is an object of the invention for the non-noble metal-basedelectro-catalyst to preclude the use of the noble metal loading that istypically required, without decreasing the electrochemical activity ascompared to pure noble metal electro-catalysts that are known in theart. The non-noble metal-based electro-catalyst compositions alsodemonstrate equivalent or improved corrosion stability in oxygenreduction reaction (ORR) and oxygen evolution reaction (OER) processes,as compared to known pure noble metal electro-catalysts. The non-noblemetal-based electro-catalyst compositions can be at least partiallydeposited or coated on a support or substrate. Suitable supports orsubstrates include a wide variety that are known in the art for use asan electrode, such as, a current collector, such as, but not limited to,titanium (Ti) foil, glassy carbon (GC) disk.

The electro-catalyst compositions can also include a dopant to thenon-noble metal component. The dopant can be selected from variouselements known in the art which can enhance electronic conductivity andstability of the composition. The dopant can be an electron donorelement. Suitable dopants include the elements in Groups III, V, VI andVII of the Periodic Table, and mixtures thereof. In certain embodiments,the non-noble metal component is doped with fluorine, chlorine, bromine,iodine, sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic,antimony, aluminum, bismuth, boron and mixtures thereof. Withoutintending to be bound by any particular theory, it is believed thatdoping increases the electronic conductivity and therefore, providesimproved stability to an electro-catalyst support.

The amount of the dopant can vary and in certain embodiments, can bepresent in an amount from greater than 0 to about 20% by weight based onthe total weight of the composition. In other embodiments, the dopantcan constitute from about 10% to about 15% by weight based on the totalweight of the composition. In preferred embodiments, about 10% by weightof fluorine is present as the dopant, based on the total weight of thecomposition.

The electro-catalyst composition can have a general formula I:

(a _(x) b _(y))O_(z) :wc  (I)

wherein, a is Mn, Cu, Zn, Sc, Fe, Co, Ti, V, Ni, Cr, Ta, Sn, Nb, W, Mo,Y, La, Ce, Nd, Er, Gd, Yb or mixtures thereof; b is Mn; O is oxygen; cis F, Cl, Br, I, S, Se, Te, N, P, As, Sb, Bi, Al, B or mixtures thereof;x and y are each a number greater than or equal to 0 and less than orequal to 2, x and y may be the same or different, when x=0, y is anumber greater than 0 and less than or equal to 2 and when y=0, x is anumber greater than 0 and less than or equal to 2; z is a number that isgreater than 0 and less than or equal to 4 (for all x and y), and w isfrom 0% by weight to about 20% by weight, based on the total weight ofthe composition.

In certain embodiments, the electro-catalyst compositions include thefollowing formulas for undoped and doped electro-catalysts of 5%, 10%and 15% by weight, respectively: Cu_(1.5)Mn_(1.5)O₄,Cu_(1.5)Mn_(1.5)O₄:5F, Cu_(1.5)Mn_(1.5)O₄:10F andCu_(1.5)Mn_(1.5)O₄:15F. In another embodiment, the electro-catalystformula is Cu_(1.5)Mn_(1.5)O_(2.75):1.25F.

In certain embodiments of the invention, the electro-catalystcompositions include binary oxide compositions, wherein the non-noblemetal component includes two non-noble metal oxides, such as, but notlimited to, copper oxide and manganese oxide. Alternatively, theelectro-catalyst compositions can include ternary oxide compositions,wherein the non-noble metal component includes three non-noble metaloxides.

The non-noble metal-based electro-catalysts of the invention can beprepared using conventional methods and apparatus known in the art. Forexample, a scalable wet chemical approach can be employed to synthesizenanostructured electro-catalyst compositions in accordance with theinvention. This approach can produce electro-catalysts with highspecific surface area for use in unsupported and supported formsutilizing carbon, undoped and doped carbon nanotubes (CNTs) withelements such as F, Cl, Br, I, S, Se, Te, N, P, As, Sb, Bi, Al, B andmixtures thereof, graphene and reduced graphene oxide, and mixturesthereof, to form a composite and an electrode.

In general, a solid solution of non-noble metal oxide nanoparticles isformed, dried and then, heat treated, to form a powder. For example, anon-noble metal oxide precursor, e.g., a salt, such as but not limitedto manganese acetate tetrahydrate, is combined with a precipitationagent or reaction agent, such as but not limited to, potassiumpermanganate, to form non-noble metal oxide nanoparticles, such as butnot limited to, manganese dioxide nanoparticles. A dopant is introducedby adding a dopant precursor, such as but not limited to, ammoniumfluoride, to form a solid solution. The solid solution is dried and heattreated to result in a doped non-noble metal oxide powder, such as butnot limited to, fluorine-doped manganese oxide powder.

The non-noble metal-based electro-catalysts prepared according to theinvention demonstrate one or more of the following advantages:

-   -   Multifunctional non-noble metal based electro-catalyst        compositions (a_(x)b_(y))O_(z): w wt. % c (a=Mn, Cu, Zn, Sc, Fe,        Co, Ti, V, Ni, Cr, Ta, Sn, Nb, W, Mo, Y, La, Ce, Nd, Er, Gd, Yb        and mixtures thereof; b=Mn, O=oxygen; c=F, Cl, Br, I, S, Se, Te,        N, P, As, Sb, Bi, Al, B and mixtures thereof; 0≤x≤2, 0≤y≤2; when        x=0, 0<y≤2, when y=0, 0<x≤2, 0<z≤4 for all x and y; and w=0-20        wt. %) with unique modified electronic structure resulting in        electro-catalytically active phase for OER in PEM based water        electrolysis and ORR in PEMFCs and DMFCs.    -   Nanostructured electro-catalyst compositions with high specific        surface area (>˜109 m²/g) synthesized using simple and easily        scalable wet chemical synthesis approach.    -   High electrical conductivity of electro-catalyst compositions        resulting in superior charge transfer kinetics, almost similar        or superior to that of the noble metal electro-catalysts.    -   Excellent onset potential of electro-catalyst compositions for        OER and ORR, similar to that of state of the art noble metal        electro-catalysts (Pt, IrO₂).    -   Remarkable electro-catalytic activity (excellent reaction        kinetics) of electro-catalyst compositions for OER and ORR,        comparable to that of state of the art noble metal        electro-catalysts (Pt, IrO₂).    -   Excellent methanol tolerance of electro-catalyst compositions,        significantly superior than that of state of the art noble metal        electro-catalysts (Pt), making electro-catalyst compositions        suitable for use in DMFC cathodes.    -   Excellent long term electrochemical stability and superior        cycling characteristics of electro-catalyst compositions under        harsh acidic operating conditions of OER and ORR, similar to        that of state of the art noble metal electro-catalysts (Pt,        IrO₂).    -   Outstanding electro-catalytic performance and unprecedented long        term stability for OER and ORR for non-noble metals based        electro-catalyst in acidic media.

The excellent electro-catalytic activity and superior long termstability of electro-catalyst compositions make the electro-catalystcompositions suitable for use in energy generation and storagetechnologies such as metal-air batteries. Thus, the electro-catalysts ofthe invention can be used as a catalyst in metal-air batteries, andhydrogen generation from solar energy and electricity-driven watersplitting.

Examples

Copper manganese oxide based electro-catalyst systems for ORR in PEMbased fuel cells (PEMFCs, DMFCs) and OER in water electrolysis wereprepared on the grounds of economic and highly efficient operation inacidic media compared to neutral and basic media. The first-principlescalculations of the total energies and electronic structures werecarried out to identify suitable Cu—Mn—O based electro-catalyst systems.Based on theoretical calculations, Cu_(1.5)Mn_(1.5)O₄ and x wt. % Fdoped Cu_(1.5)Mn_(1.5)O₄ (x=0, 5, 10, 15) were explored as highly activeand durable electro-catalysts, expected to possess unique electronicstructure resulting in adsorption and desorption of reactionintermediates similar to that of Pt for ORR and IrO₂ for OER. Fluorine(F) was used as dopant for Cu_(1.5)Mn_(1.5)O₄ to improve electronicconductivity of Cu_(1.5)Mn_(1.5)O₄, on the grounds of ubiquitous use ofF in transparent conductive oxides for solar cells, heat mirrors, andthe like, as well as the role of F in improving electrochemical activityof (Ir,Sn,Nb)O₂ systems. The synthesis of F-doped Cu_(1.5)Mn_(1.5)O₄provides an opportunity for tailoring electronic structure, physical,electronic and electro-catalytic properties of MnO_(x). Thus, thisexample provides the physical characterization and electrochemicalperformance of nanostructured Cu_(1.5)Mn_(1.5)O₄:x wt. % F (x=0, 5, 10,15) electro-catalysts for OER in PEM water electrolysis and ORR inPEMFCs and DMF Cs.

For a computational study, a qualitative evaluation of theelectrochemical activity of electro-catalysts was conducted. There wasshown the existence of a simple descriptor for determining the surfacecatalytic activity of the electro-catalysts. This descriptor was definedas a gravity center of the transition metal d-band Ed usually located inthe vicinity of the Fermi level. An optimal position of the d-bandcenter provides an optimal interaction between the catalytic surface andvarious species participating in the catalytic reactions predominantlyoccurring on the surface leading to the expected maximum catalyticactivity. Thus, such an optimal interaction allows the reactants andproducts to both adsorb at the surface and desorb most efficiently.Hence, an adjustment of the d-band center position with respect to theFermi level is likely critical in designing novel highly activeelectro-catalysts discussed herein.

The projected d-band densities of electronic states together withcorresponding centers of these zones were obtained for pure Pt, IrO₂,Cu_(1.5)Mn_(1.5)O₄, and Cu_(1.5)Mn_(1.5)O_(2.75)F_(1.25) (containing˜9.7 wt. % of F). The d-band positions of state of the artelectro-catalysts Pt and IrO₂ served as a benchmark for the optimalcatalytic activity of the designed electro-catalysts. Calculationsshowed the same d-band center positions relative to the Fermi level ofboth Pt and IrO₂— at ˜(−1.33 eV) suggesting similar interaction betweenthe catalytic surface and various intermediate species in both ORR andOER, because these reactions involve virtually the same intermediatespecies, but in the opposite directions. Thus, the closer thecorresponding d-band center of the electro-catalyst to d-band centerposition of Pt or IrO₂, the better the overall catalytic activity of theelectro-catalyst. Thus, the equivalence of the electronic structure ofthe F-doped copper manganese oxide electro-catalyst composition with thePt and IrO₂ electro-catalysts, was demonstrated.

The calculated projected 3d-electronic density of states of Cu and Mnelements in Cu_(1.5)Mn_(1.5)O₄ showed the d-band center located at −1.05eV vs Fermi level, which was slightly above Pt or IrO₂ benchmark line,however very near to it, indicating relatively high catalytic activityof Cu_(1.5)Mn_(1.5)O₄. An introduction of F into Cu_(1.5)Mn_(1.5)O₄modified an overall electronic structure such that formation ofhybridized F2p-Mn3d electronic states below −7 eV shifted the d-bandcenter downward to −1.45 eV, which was slightly below that of Pt or IrO₂(−1.33 eV). Assuming linear shift of the d-band center during increaseof F-content, the most optimal F-content bringing the overall d-bandcenter of F-doped Cu_(1.5)Mn_(1.5)O₄ right to the Pt or IrO₂ benchmarkposition should be approximately 8-10 wt. % of F, which is similar toexperimentally determined optimal F content of 10 wt. % (discussedlater). Thus, F-doping in Cu_(1.5)Mn_(1.5)O₄ resulted in themodification of the electronic structure in general and shift of thed-band center position to Pt or IrO₂ in particular, thereby improvingthe overall catalytic activity of Cu_(1.5)Mn_(1.5)O₄.

The XRD patterns of chemically synthesized Cu_(1.5)Mn_(1.5)O₄ andCu_(1.5)Mn_(1.5)O₄:F of different F content, shown in FIG. 1a , showsthe single phase cubic structure with peaks corresponding toCu_(1.5)Mn_(1.5)O₄ (JCPDS card no: 70-0260) without any peaks ofsecondary phase, suggesting incorporation of F in lattice ofCu_(1.5)Mn_(1.5)O₄. The lattice parameters of Cu_(1.5)Mn_(1.5)O₄ andCu_(1.5)Mn_(1.5)O₄:F of all compositions is a˜0.827 nm and molar volumeof 85.16 cm³/mol, which is consistent with the reported literature valueand suggests no significant effect of F doping on molar volume ofCu_(1.5)Mn_(1.5)O₄:F. This can be potentially due to comparable ionicradius of O⁻² (125 pm) and F⁻¹ (120 pm). The effective crystallite sizeof Cu_(1.5)Mn_(1.5)O₄ and Cu_(1.5)Mn_(1.5)O₄:F of different F content,(calculated using the Scherrer formula) was 8-10 nm indicatingnano-crystalline nature of Cu_(1.5)Mn_(1.5)O₄:F with minimal effect of Fdoping on crystallite size of Cu_(1.5)Mn_(1.5)O₄. The measured BETsurface area of Cu_(1.5)Mn_(1.5)O₄ and Cu_(1.5)Mn_(1.5)O₄:F was 109m²/g, which can be due to minimal effect of F-doping on particle size ofall compositions as discussed (see Table 1).

TABLE 1 Results of electrochemical characterization for OER. Currentdensity for Onset OER at potential for 1.55 V Tafel BET surface OER (vsRHE) R_(s) R_(e) R_(ct) slope Electro-catalyst area (m²/g) (V vs RHE)(mA/cm²) (Ω · cm²) (Ω · cm²) (Ω · cm²) (mV/dec) Cu_(1.5)Mn_(1.5)O₄ 1091.43 6.36 16.38 5 44.9 66.8 Cu_(1.5)Mn_(1.5)O₄:5F 109 1.43 7.32 16.314.71 29.2 65.7 Cu_(1.5)Mn_(1.5)O₄:10F 109 1.43 9.15 16.35 3.5 15.15 60Cu_(1.5)Mn_(1.5)O₄:15F 109 1.43 5.63 16.36 4.95 47.2 69.1 IrO₂ 191 1.437.74 16.35 3.65 17.9 —

The SEM image along with the EDX pattern of Cu_(1.5)Mn_(1.5)O₄:10Fshowed the presence of Cu, Mn and O. The quantitative elementalcomposition analysis of Cu_(1.5)Mn_(1.5)O₄:10F obtained by EDX confirmedthat the measured elemental composition of Cu and Mn was close to thenominal composition. The elemental x-ray maps of Cu, Mn and O ofCu_(1.5)Mn_(1.5)O₄:10F showed homogeneous distribution of Cu, Mn and Owithin the particles without segregation at any specific site. Thebright field TEM image of a representative compositionCu_(1.5)Mn_(1.5)O₄:10F (FIG. 1b ), showed nanometer sized particles inthe size range 8-10 nm which was consistent with the XRD analysis. TheHRTEM image of Cu_(1.5)Mn_(1.5)O₄:10F showed lattice fringes with aspacing of ˜0.249 nm which corresponded with the (113) inter-planerspacing of cubic Cu_(1.5)Mn_(1.5)O₄:10F determined from XRD analysis.The elemental oxidation state was studied by conducting x-rayphotoelectron spectroscopy (XPS) on the electro-catalysts. The XPSspectrum of Cu of Cu_(1.5)Mn_(1.5)O₄:0F showed a broad peak between ˜940eV-945 eV corresponding to Cu′ satellites and peaks centered at 931 eVand 934 eV suggesting the presence of monovalent (Cu⁺) and divalentcopper (Cu²⁺), respectively. The XPS spectrum of Mn indicated presenceof both Mn′ and Mn⁴⁺. The presence of F could not be ascertained by XPSanalysis. However, a positive shift of ˜0.4 eV in Cu 2p_(3/2) and Mn2p_(3/2) peaks of Cu_(1.5)Mn_(1.5)O₄:10F was seen compared to that ofCu_(1.5)Mn_(1.5)O₄, indicating stronger binding potentially due tohigher electro-negativity of fluorine incorporated into the lattice. Thepresence of F was confirmed using NMR spectroscopy. FIG. 1c showsdramatic loss of ¹⁹F NMR signal for Cu_(1.5)Mn_(1.5)O₄:5F andCu_(1.5)Mn_(1.5)O₄:10F presumably due to large ¹⁹F-electron hyperfineinteractions due to the unpaired electrons from the paramagnetic Cu andMn centers, indicating position of F atoms close to Mn/Cu in thelattice. FIG. 1c shows clear ¹⁹F resonances at ˜(−110 ppm) forCu_(1.5)Mn_(1.5)O₄:15F and Cu_(1.5)Mn_(1.5)O₄:20F samples indicating thediamagnetic nature, and also suggesting the position of F atoms fartheraway from the metal center. Thus, NMR results not only confirmedpresence of F in Cu_(1.5)Mn_(1.5)O₄:F but also provided informationabout its proximity to metal centers.

The onset potential of OER for Cu_(1.5)Mn_(1.5)O₄ andCu_(1.5)Mn_(1.5)O₄:F of all compositions was ˜1.43±0.001 V (vs RHE),which is similar to that of in-house synthesized as well as commerciallyobtained IrO₂ (FIG. 2a-b and Table 1). This suggests similar reactionpolarization of Cu_(1.5)Mn_(1.5)O₄:F of different F content to that ofthe in-house synthesized IrO₂. Cu_(1.5)Mn_(1.5)O₄:F of different Fcontent exhibited the peak potential of reduction of surface oxides of˜0.75±0.001 V (vs RHE) similar to that of Pt/C (FIG. 2d-e ). The similarpeak potential of reduction of surface oxides of Cu_(1.5)Mn_(1.5)O₄:Fand Pt/C suggests similar binding strength of oxygen containing species(OH, O, O₂) on the surface of each electro-catalyst and thus, similarreaction polarization for ORR of Cu_(1.5)Mn_(1.5)O₄:F and Pt/C. Theseresults indicate unique electronic structure of Cu_(1.5)Mn_(1.5)O₄:F ofdifferent F content offering lower reaction polarization which issimilar to that of noble electro-catalysts.

The electrolyte solution resistance (R_(s)), electrode resistance(R_(e)) and bubble resistance (R_(bub)) are mainly responsible forlinear nature of polarization curve and non-linearity in Tafel plot.Thus, to study the inherent electrochemical activity ofelectro-catalysts, ohmic resistance (R_(Ω)) correction(iR_(Ω)=iR_(s)+iR_(e)) was conducted in polarization and cyclicvoltammogram (CV) curves. The values of R_(s) and R_(e) of the differentelectro-catalysts were obtained from electrochemical impedancespectroscopy (EIS) measurements (discussed later) and given in Table 1and Table 2.

TABLE 2 Results of electrochemical characterization for ORR. Currentdensity for ORR Tafel slope at 0.9 V (mV/dec) (vs RHE) R_(Ω) R_(ct) inin Electro-catalyst (mA/cm²) (Ω · cm²) (Ω · cm²) LCR HCRCu_(1.5)Mn_(1.5)O₄ 0.44 16.5 31.5 75 130 Cu_(1.5)Mn_(1.5)O₄:5F 0.7 16.4528.55 72 127 Cu_(1.5)Mn_(1.5)O₄:10F 1.15 16.4 15.6 68 123Cu_(1.5)Mn_(1.5)O₄:15F 0.35 16.38 41.62 79 141 Pt/C 1.26 16.39 9.2 — —

The current density for in-house synthesized IrO₂ electro-catalyst(total loading=0.15 mg/cm²) is ˜7.74±0.0001 mA/cm² at 1.55 V (vs RHEtypical potential selected for assessing electrochemical activity forOER) (FIG. 2a-c and Table 1). Cu_(1.5)Mn_(1.5)O₄, Cu_(1.5)Mn_(1.5)O₄:5F,Cu_(1.5)Mn_(1.5)O₄:10F and Cu_(1.5)Mn_(1.5)O₄:15F (total loading=1mg/cm²) showed excellent electro-catalytic activity for OER with currentdensity of ˜6.36±0.001 mA/cm², ˜7.32±0.001 mA/cm², ˜9.15±0.0001 mA/cm²and ˜5.63±0.001 mA/cm² at 1.55 V (vs RHE), respectively (FIG. 2a-c andTable 1). Thus, Cu_(1.5)Mn_(1.5)O₄, Cu_(1.5)Mn_(1.5)O₄:5F,Cu_(1.5)Mn_(1.5)O₄:10F and Cu_(1.5)Mn_(1.5)O₄:15F showed remarkableelectrochemical activity, i.e., 83%, 95%, 118% and 73% of that ofin-house synthesized IrO₂ (FIG. 2c ). The reaction kinetics ofCu_(1.5)Mn_(1.5)O₄:F was studied by conducting EIS to determine R_(s),R_(e) and charge transfer resistance (R_(d)). The decrease in electroderesistance (R_(e)) with increase in F content up to 10 wt. % doped inCu_(1.5)Mn_(1.5)O₄ lattice can be due to the improved electronicconductivity of Cu_(1.5)Mn_(1.5)O₄:F up to 10 wt. % F followed bydecrease in electronic conductivity (increase in R_(e)) for 15 wt. % Fcontent (Table 1). R_(ct) determined from the diameter of semi-circle inhigh frequency region of EIS plot and Tafel slope (Table 1) decreasewith increase in F content with the lowest R_(ct) and Tafel slopeobtained for Cu_(1.5)Mn_(1.5)O₄:10F (˜15.15±0.0001 Ω·cm² and ˜60±0.0001mV/dec) suggesting improvement in reaction kinetics (decrease inactivation polarization) with increase in F content up to 10 wt. % F.The R_(ct) and Tafel slope increase after continued increase in Fcontent beyond 10 wt. % potentially due to poor reaction kinetics andalso decrease in electronic conductivity (due to increase in R_(e))(Table 1). The Tafel slope of Cu_(1.5)Mn_(1.5)O₄:10F (˜60±0.0001 mV/dec)indicate two electron pathway for OER (Table 1). It is noteworthy thatR_(ct) for Cu_(1.5)Mn_(1.5)O₄:10F (˜15.15±0.0001 Ω·cm²) is lower thanthat of the in-house synthesized IrO₂ (˜17.9±0.0001 Ω·cm²) indicatingexcellent reaction kinetics for Cu_(1.5)Mn_(1.5)O₄:10F resulting incomparable electrochemical activity (i.e., current density) to that ofIrO₂ (FID. 2 a-c and Table 1).

The electrochemical activity for ORR was studied by comparing currentdensity at 0.9 V (vs RHE, typical potential used for study ofelectrochemical activity for ORR) in iR_(Ω) corrected polarizationcurves obtained in O₂-saturated 0.5 M H₂SO₄ electrolyte solution at 26°C. (FIG. 3a ). Cu_(1.5)Mn_(1.5)O₄:F exhibited excellent electrochemicalactivity for ORR with onset potential of ORR similar to that of Pt/C(˜1±0.001 V vs RHE) mainly due to similar reaction polarization, asdiscussed earlier (FIG. 2d-e and FIG. 3a ). The current density at 0.9 V(vs RHE) for Pt/C is ˜1.26±0.0001 mA/cm² (Table 2). Cu_(1.5)Mn_(1.5)O₄,Cu_(1.5)Mn_(1.5)O₄:5F, Cu_(1.5)Mn_(1.5)O₄:10F and Cu_(1.5)Mn_(1.5)O₄:15Fexhibited current density of ˜0.44±0.0001 mA/cm², ˜0.7±0.001 mA/cm²,˜1.15±0.0001 mA/cm² and ˜0.35±0.001 mA/cm² at 0.9 V (vs RHE) (FIG. 3a-band Table 2). Thus, Cu_(1.5)Mn_(1.5)O₄, Cu_(1.5)Mn_(1.5)O₄:5F,Cu_(1.5)Mn_(1.5)O₄:10F and Cu_(1.5)Mn_(1.5)O₄:15F exhibited 35%, 56%,92% and 28% electrochemical activity for ORR of that of Pt/C,respectively (FIG. 3b ). Thus, electrochemical activity increases uponF-doping in Cu_(1.5)Mn_(1.5)O₄ up to 10 wt. % F content mainly due todecrease in R_(ct) and Tafel slope (decrease in activation polarization)(Table 2) with the lowest obtained for Cu_(1.5)Mn_(1.5)O₄:10F(˜15.6±0.001 Ω·cm², ˜68±0.001 mV/dec in low current region (LCR) and˜123±0.001 mV/dec in high current region (HCR)) implying fast reactionkinetics for Cu_(1.5)Mn_(1.5)O₄:10F and then, the increase in bothvalues for continued increase in F content above 10 wt. % indicatingpoor reaction kinetics. The number of electrons involved in ORR forCu_(1.5)Mn_(1.5)O₄:10F determined from the Koutechy-Levich plot is 3.88,suggesting the desired direct four electron pathway of ORR forCu_(1.5)Mn_(1.5)O₄:10F. In addition, Cu_(1.5)Mn_(1.5)O₄:10F exhibitedexcellent methanol tolerance for use as cathode electro-catalyst inDMFCs, superior to that of the standard Pt/C electro-catalyst.

The polarization curve of a single PEMFC full cell made ofCu_(1.5)Mn_(1.5)O₄:10F (total loading=0.3 mg/cm²) as cathodeelectro-catalyst and commercial Pt/C (Alfa Aesar) as anodeelectro-catalyst (Pt loading=0.2 mg_(Pt)/cm²) showed maximum powerdensity of ˜550 mW/cm² which is ˜56% of that obtained using Pt/C (Ptloading=0.3 mg_(Pt)/cm²) as cathode electro-catalyst (˜990 mW/cm²). Thisis a hallmark finding as the DOE recommendable power density (˜550mW/cm²) was obtained using the novel electro-catalyst(Cu_(1.5)Mn_(1.5)O₄:10F) devoid of any precious or noble metals.

The long term electrochemical stability of Cu_(1.5)Mn_(1.5)O₄:10F andin-house synthesized IrO₂ was studied by performing chronoamperometry(CA) test wherein, the electrode was maintained at a constant voltage of˜1.55 V (vs RHE) in 0.5 M H₂SO₄ electrolyte for 24 h and loss inelectro-catalytic activity (i.e., current density) for OER was studied(FIG. 3c ). The minimal loss in current density in CA curve (similar tothat of IrO₂) (FIG. 3c ) along with minimal loss in electrochemicalperformance for OER after 24 h of CA test, that is negligible loss incurrent density for ORR after stability test for 6000 cycles (FIG. 3d ),no detection of Cu/Mn in ICP analysis conducted on the electrolytesolution after stability test for OER and ORR, negligible loss in singlePEMFC performance after 48 h of operation suggests the excellent longterm electrochemical stability for OER and ORR of Cu_(1.5)Mn_(1.5)O₄:10Fin acidic media.

In summary, there was demonstrated a hallmark development of novelnon-noble electro-catalyst (Cu_(1.5)Mn_(1.5)O₄:F) with zero noble metalcontent with unique atomic/molecular structure exhibiting remarkablestability and outstanding electrochemical activity obtained forCu_(1.5)Mn_(1.5)O₄:10F which is comparable to that of IrO₂ for OER and˜92% of that of Pt/C for ORR, respectively. Hence,Cu_(1.5)Mn_(1.5)O₄:10F provides support for replacing Pt, IrO₂ and thus,this is a fundamental breakthrough in the pursuit of completelynon-precious electro-catalyst for economic and efficient hydrogenproduction from PEM water electrolysis with proficient power generationfrom fuel cells (PEMFCs, DMFCs).

Methods

Preparation of Cu_(1.5)Mn_(1.5)O₄:x Wt. % F (x=0, 5, 10, 15)Nanoparticles (NPs)

Synthesis of MnO₂ NPs

Manganese acetate tetrahydrate (Mn(CH₃COO)₂.4H₂O, 1.5 mmol, 99.99%,Aldrich) was dissolved in 25 mL D.I. water purified by the Milli-Qsystem (18 MΩcm deionized water, Milli-Q Academic, Millipore).Separately, KMnO₄ (1 mmol) was dissolved in 25 mL D.I. water. KMnO₄solution was then added to Mn(CH₃COO)₂.4H₂O solution with vigorousstirring, which immediately formed a brown slurry. After ˜1 h stirring,the precipitate was obtained by filtration and then thoroughly washedwith D.I. water, followed by drying at 60° C. for 2 h.

Synthesis of Cu_(1.5)Mn_(1.5)O₄:x wt. % F NPs

For the preparation of Cu_(1.5)Mn_(1.5)O₄NPs, copper chloride dihydrate(CuCl₂.2H₂O, ≥99%, Aldrich) was dissolved in D.I. water. The preparationof Cu_(1.5)Mn_(1.5)O₄ was performed by soaking the suitable amount ofcopper chloride dihydrate (CuCl₂.2H₂O, ≥99%, Aldrich) aqueous solutionon the surface of the as-prepared MnO₂ NPs. For synthesis ofCu_(1.5)Mn_(1.5)O₄:F, ammonium fluoride (NH₄F, 98%, Alfa Aesar)dissolved in D.I. water was also introduced after the addition ofCuCl₂.2H₂O solution. The solution was then dried in an alumina cruciblein drying oven at 60° C. for 2 h, followed by heat treatment in air at500° C. for 4 h in order to form Cu_(1.5)Mn_(1.5)O₄:F of different Fcontent. Cu_(1.5)Mn_(1.5)O₄:x wt. % F (x=0, 5, 10, 15) are denoted asCu_(1.5)Mn_(1.5)O₄:0F, Cu_(1.5)Mn_(1.5)O₄:5F, Cu_(1.5)Mn_(1.5)O₄:10F andCu_(1.5)Mn_(1.5)O₄:15F herein, respectively.

Materials Characterization

The phase analysis of electro-catalyst materials was carried out byx-ray diffraction (XRD) using Philips XPERT PRO system employing CuK_(α)(λ=0.15406 nm) radiation at an operating voltage and current of 45 kVand 40 mA, respectively. The XRD peak profile of Cu_(1.5)Mn_(1.5)O₄:F ofdifferent F content was analyzed using the Pseudo-Voigt function todetermine the Lorentzian and Gaussian contribution of the peak. Theintegral breadth of the Lorentzian contribution, determined from peakprofile analysis using the single line approximation method aftereliminating the instrumental broadening and lattice strain contribution,was used in the Scherrer formula to calculate the particle size ofCu_(1.5)Mn_(1.5)O₄:F of different composition. The lattice parameter andmolar volume of synthesized Cu_(1.5)Mn_(1.5)O₄:F of differentcomposition have been calculated using the least square refinementtechniques.

Scanning electron microscopy (SEM) was carried out to investigate themicrostructure of Cu_(1.5)Mn_(1.5)O₄:F. Quantitative elemental analysisand distribution of elements (by elemental x-ray mapping) was obtainedby utilizing the energy dispersive x-ray spectroscopy (EDX) analyzerattached with the SEM machine. Philips XL-30FEG equipped with an EDXdetector system comprising of an ultrathin beryllium window and Si(Li)detector operating at 20 kV was used for the elemental and x-ray mappinganalysis. Transmission electron microscopy and high resolutiontransmission electron microscopy (HRTEM) analysis was conducted usingthe JEOL JEM-2100F microscope to investigate the particle size andmorphology of electro-catalyst materials. The specific surface area(SSA) of electro-catalyst materials was determined by conductingnitrogen adsorption-desorption studies and analyzing the data using theBrunauer-Emmett-Teller (BET) isotherms. The powder was first vacuumdegassed and then tested using a Micromeritics ASAP 2020 instrument.Multipoint BET specific surface areas analyses have been conducted andindicated for the synthesized electro-catalyst powders (Table 1).

X-ray photoelectron spectroscopy (XPS) was used to investigate thevalence states of Cu and Mn ions of Cu_(1.5)Mn_(1.5)O₄:F. XPS analysiswas carried out using a Physical Electronics (PHI) model 32-096 X-raysource control and a 22-040 power supply interfaced to a model 04-548X-ray source with an Omni Focus III spherical capacitance analyzer(SCA). The system is routinely operated within the pressure range of10⁻⁸ to 10⁻⁹ Torr (1.3×10⁻⁶ to 1.3×10⁻⁷ Pa). The system was calibratedin accordance with the manufacturer's procedures utilizing thephotoemission lines E_(b) of Cu 2p_(3/2). (932.7 eV), E_(b) of Au4f_(7/2) (84 eV) and E_(b) of Ag 3d_(5/2) (368.3 eV) for a magnesiumanode. All the reported intensities are experimentally determined peakareas divided by the instrumental sensitivity factors. Charge correctionwas obtained by referencing the adventitious C 1s peak to 284.8 eV. Thepresence of F in synthesized electro-catalyst materials was confirmed bycollecting ¹⁹F NMR spectra on an Avance 500 MHz Wide Bore NMRspectrometer using a 3.2 mm CP-MAS probe at a spinning speed of 14 kHz.

Electrochemical Characterization as OER Electro-Catalyst

Electrochemical characterization of Cu_(1.5)Mn_(1.5)O₄:X wt. % F (x=0,5, 10, 15) nanoparticles (NPs) was performed at 40° C. (using a FisherScientific 910 Isotemp refrigerator circulator) on a VersaSTAT 3(Princeton Applied Research) electrochemical workstation using a threeelectrode configuration in the electrolyte solution of 0.5 M sulfuricacid (H₂SO₄) which also served as a proton source. Prior toelectrochemical testing, oxygen from the electrolyte solution wasexpelled by purging electrolyte solution with ultra-high-purity (UHP)-N₂gas. The electro-catalyst ink was prepared using 85 wt. %electro-catalyst and 15 wt. % Nafion 117 (5 wt. % solution in loweraliphatic alcohols, Aldrich) and further sonicated. The workingelectrodes were prepared by spreading the electro-catalyst ink ofCu_(1.5)Mn_(1.5)O₄:x wt. % F (x=0, 5, 10, 15) on porous Ti foil (AlfaAesar) with the total loading of 1 mg on 1 cm² area. A Pt wire (AlfaAesar, 0.25 mm thick, 99.95%) was used as the counter electrode andmercury/mercurous sulfate (Hg/Hg₂SO₄) electrode (XR-200, Hach) that hasa potential of +0.65 V with respect to normal hydrogen electrode (NHE)was used as the reference electrode. The electrochemical performance ofCu_(1.5)Mn_(1.5)O₄:F for OER is compared with state of the art IrO₂electro-catalyst in this study. Hence, the electrochemical performanceof in-house synthesized IrO₂ electro-catalyst was analyzed with totalloading of 0.15 mg on 1 cm² area under identical operating conditions.All the potential values in this study are reported with respect toreversible hydrogen electrode (RHE), calculated from the formula:E_(RHE)=E_(Hg/Hg2 SO4)+E⁰ _(Hg/Hg2SO4)+0.059 pH, wherein E_(RHE) is thepotential versus RHE. E_(Hg/Hg2SO4) is the potential measured againstthe Hg/Hg₂SO₄ reference electrode. E⁰ _(Hg2SO4) is the standardelectrode potential of Hg/Hg₂SO₄ reference electrode (+0.65 V vs NHE).

The electrochemical activity of electro-catalysts for OER was determinedby conducting polarization measurements in 0.5 M H₂SO₄ electrolytesolution at a scan rate of 5 mV/sec at 40° C. Polarization curves ofdifferent electro-catalysts were iR_(Ω) corrected (R_(Ω), the ohmicresistance was determined from electrochemical impedance spectroscopyanalysis discussed later). The current density at 1.55 V (vs RHE, whichis typical potential selected for comparison of electrochemical activityof electro-catalyst for OER) in iR_(Ω) corrected polarization curves wasused for comparison of electrochemical performance of differentelectro-catalysts. The Tafel plot after iR_(Ω) correction given by theequation η=a+b log i (plot of overpotential η vs log current, log i) wasused to determine Tafel slope (b), which was further used to study thereaction kinetics for all electro-catalysts.

Electrochemical Impedance Spectroscopy

The ohmic resistance (R_(Ω)) (which includes resistance from componentslike electrolyte, electrode) and the charge transfer resistance (R_(ct))of electro-catalysts were determined from electrochemical impedancespectroscopy (EIS). The frequency range of 100 mHz-100 kHz (Amplitude=10mV) was used for EIS, which was carried out using the electrochemicalwork station (VersaSTAT 3, Princeton Applied Research) in 0.5 M H₂SO₄electrolyte solution at 40° C. at 1.55 V (vs RHE which is typicalpotential used for assessing of electrochemical activity ofelectro-catalyst for OER) using total loading of 1 mg/cm² forCu_(1.5)Mn_(1.5)O₄:F of different F content and 0.15 mg/cm² for in-housesynthesized IrO₂. Impedance data for OER has been modeled by using theZView software from Scribner Associates employing theR_(s)(R_(e)Q_(l))(R_(ct)Q_(dl)) circuit model to determine:R_(s)=Resistance faced at high frequency due to charge transfer inelectrolyte solution; R_(e)=Resistance for electron transfer from theelectrode to current collector (Ti foil); R_(ct)=Charge transferresistance (i.e., polarization resistance); Q_(l)=Constant phaseelement; and Q_(dl)=Contribution from both double layer capacitance andpseudocapacitance. The ohmic resistance (R_(Ω)) obtained from the EISwas used for iR_(Ω) (iR_(s)+iR_(e)) correction in the polarizationcurves of electro-catalysts.

Electrochemical Stability Test

The electrochemical stability of Cu_(1.5)Mn_(1.5)O₄:10F electro-catalyst(total loading=1 mg/cm²) for long term operation was studied byconducting chronoamperometry (CA) (current vs time) for 24 h using 0.5 MH₂SO₄ as the electrolyte solution at 40° C. at constant voltage of 1.55V (vs RHE). For comparison, CA test was also conducted for in-housesynthesized IrO₂ (total loading=0.15 mg/cm²). The electrolyte (H₂SO₄)solution collected after 24 h of CA testing of electro-catalyst materialwas analyzed using inductively coupled plasma optical emissionspectroscopy (ICP-OES, iCAP 6500 duo Thermo Fisher) to determine theconcentration of elements leached out in the electrolyte solution fromthe electrode. This is important as the concentration of elements in theelectrolyte solution can be correlated to the electrochemical stabilityof electro-catalyst.

Electrochemical Characterization as ORR Electro-Catalyst

The electrochemical characterization was carried out using a rotatingdisk electrode (RDE) setup. The electro-catalyst ink (85 wt. %electro-catalyst and 15 wt. % Nafion 117) was sonicated and applied to aglassy carbon (GC) disk (geometric area=0.19 cm²). After solventevaporation, the GC surface had a thin layer of electro-catalyst, whichserved as the working electrode. The total loading ofCu_(1.5)Mn_(1.5)O₄:x wt. % F (x=0, 5, 10, 15) was 50 μg/cm². Theelectrochemical performance of Cu_(1.5)Mn_(1.5)O₄:F for ORR is comparedwith state of the art Pt/C electro-catalyst in this study. Hence, theelectrochemical performance of commercially obtained 40% Pt/Celectro-catalyst (Alfa Aesar) was analyzed with Pt loading of 30 μg_(Pt)on 1 cm² area under identical operating conditions. A Pt wire (AlfaAesar, 0.25 mm thick, 99.95%) was used as the counter electrode andHg/Hg₂SO₄ was used as the reference electrode (+0.65 vs NHE).

Electrochemical characterization was conducted in an electrochemicalworkstation (VersaSTAT 3, Princeton Applied Research) using a threeelectrode cell configuration at 26° C. (using a Fisher Scientific 910Isotemp refrigerator circulator). The cyclic voltammetry was conductedin N₂-saturated 0.5 M H₂SO₄ electrolyte solution by scanning thepotential between 0 V (vs RHE) and 1.23 V (vs RHE) at scan rate of 5mV/sec. ORR measurement was carried out by performing polarizationstudies in O₂-saturated 0.5 M H₂SO₄ electrolyte solution at 26° C. usingrotation speed of 2500 rpm and scan rate of 5 mV/sec. Polarization wasconducted in multiple small potential steps on the RDE to reduce thecontribution by the charging current and the current measurement wasperformed at the end of each step.^(3,4) The current density at 0.9 V(vs RHE, the typical potential for assessing electrochemical activity ofelectro-catalysts for ORR) in iR_(Ω) corrected (R_(Ω), the ohmicresistance was determined from electrochemical impedance spectroscopyanalysis described below) polarization curves of electro-catalysts wasused to compare the electrochemical performance of the differentelectro-catalyst materials. The Tafel plot after iR_(Ω) correction givenby the equation η=a+b log i (plot of overpotential η vs log current, logi) and the corresponding Tafel slope (b) has been used to study thereaction kinetics of ORR. The Koutechy-Levich equation was used todetermine the number of electrons (n) involved in the reaction:i⁻¹=i_(k) ⁻¹+i_(L) ⁻¹, wherein i_(L)=0.620nFA_(e)D₀^(2/3)ω^(1/2)ν^(−1/6)C_(o)*. Here, i_(L) is the limiting current (A,Ampere) at 0.6 V (vs RHE), i_(k) is the kinetic current (A, Ampere)observed in the absence of any mass transfer limitation, F is Faradayconstant (96489 C/mol), A_(e) is the geometric area of electrode (0.19cm²), Do is diffusivity of O₂ in 0.5 M H₂SO₄ solution (2.2×10⁻⁵cm²/sec), co is rotation speed (rad/sec), ν is the kinematic viscosityof water (0.01 cm²/sec) and C_(o)* is the saturated concentration of O₂in 0.5 M H₂SO₄ solution (0.25×10⁻⁶ mol/cm³).

Electrochemical Impedance Spectroscopy

Electrochemical impedance spectroscopy (EIS) was carried out todetermine the ohmic resistance (R_(Ω)) (which includes the resistance ofvarious components including, electrolyte and electrode) and chargetransfer resistance (or polarization resistance) (R_(ct)) ofelectro-catalysts. EIS has been conducted in the frequency range of 100mHz-100 kHz (Amplitude=10 mV) at 0.9 V (vs RHE which is typicalpotential for assessing electro-catalyst activity for ORR) inO₂-saturated 0.5 M H₂SO₄ solution at 26° C. using the electrochemicalwork station (VersaSTAT 3, Princeton Applied Research). Theexperimentally obtained EIS plot was fitted using the ZView softwarefrom Scribner Associates with a circuit model R_(Ω)(R_(ct)Q_(l)W_(o)),where Q_(l) is constant phase element and W_(o) is open circuit terminusWarburg element. R_(Ω) was used for ohmic loss correction (iR_(Ω)) inthe polarization curves of electro-catalysts.

Methanol Tolerance Test

Methanol tolerance test was carried out for electro-catalyst byperforming polarization in O₂-saturated 0.5 M H₂SO₄ electrolyte solutionwith presence of 1 M methanol at rotation speed of 2500 rpm and scanrate of 5 mV/sec at 26° C.

Electrochemical Stability/Durability Test

The electrochemical stability/durability of electro-catalyst for longterm operation was studied by performing cyclic voltammetry by scanningpotential between 0.6 V (vs RHE) and 1.23 V (vs RHE) in N₂-saturated 0.5M H₂SO₄ electrolyte solution at 26° C. at scan rate of 5 mV/sec for 6000cycles, followed by conducting polarization in O₂-saturated 0.5 M H₂SO₄solution after 6000 cycles at 26° C. using rotation speed of 2500 rpmand scan rate of 5 mV/sec. Elemental analysis of the electrolyte wasperformed after 6000 cycles by inductively coupled plasma opticalemission spectroscopy (ICP-OES, iCAP 6500 duo Thermo Fisher) todetermine the amount of elements leached out into the electrolytesolution from the electrode providing information about theelectrochemical stability of the electro-catalyst.

Membrane Electrode Assembly (MEA) Preparation and Single Cell TestAnalysis

The anode and cathode electro-catalyst ink was prepared consisting of 85wt. % electro-catalyst and 15 wt. % Nafion 117 solution (5 wt. %solution in lower aliphatic alcohols, Sigma-Aldrich). For anode, Ptloading of commercial 40% Pt/C (Alfa Aesar) electro-catalyst was 0.2mg_(Pt)/cm². For cathode, the total loading of 0.3 mg/cm² was used forCu_(1.5)Mn_(1.5)O₄:10F electro-catalyst. For comparison, 40% Pt/C (AlfaAesar) was also studied as cathode electro-catalyst in single cell testusing Pt loading of 0.3 mg_(Pt)/cm². The electrodes were prepared byspreading the electro-catalyst ink on teflonized carbon paper. For thesingle cell testing, a membrane electrode assembly was fabricated byusing a Nafion 115 membrane which was sandwiched between the anode andcathode. The Nafion 115 membrane was pretreated first with 3 wt. %hydrogen peroxide solution to its boiling point to oxidize any organicimpurities. Subsequently, it was boiled in D.I. water followed byimmersion in boiling 0.5 M sulfuric acid solution to eliminateimpurities. Finally, it was washed multiple times in D.I water to removeany traces of remnant acid. This membrane was then stored in D.I. waterto avoid dehydration. The sandwiching of Nafion 115 membrane betweenanode and cathode was carried out by hot-pressing in a 25T hydrauliclamination hot press with dual temperature controller (MTI Corporation)at a temperature of 125° C. and pressure of 40 atm applied for 30 sec toensure good contact between the electrodes and the membrane. This MEAwas then used in the single cell test analysis, carried out for 48 hoursusing fuel cell test set up obtained from Electrochem Incorporation at80° C. and 0.1 MPa with UHP-H₂ (200 ml/min) and UHP-O₂ (300 ml/min) asreactant gases.

Computational Methodology

The overall electro-catalytic activity of the Cu_(1.5)Mn_(1.5)O₄electro-catalyst was expected to depend on its electronic structure. Theeffect of compositions on the electronic structure and theelectro-catalytic activity of the electro-catalyst can bewell-understood from the theoretical considerations. The computationalcomponent of this study was to investigate the electronic structure ofpure Cu_(1.5)Mn_(1.5)O₄ and F-doped Cu_(1.5)Mn_(1.5)O₄. The totalenergy, electronic and optimized crystal structures as well as total andprojected densities of electronic states for pure and F-dopedCu_(1.5)Mn_(1.5)O₄ were calculated using the first principles approachwithin the density functional theory (DFT). The electronic structure ofthe stable surfaces for all electro-catalysts were calculated in thisstudy and the positions of corresponding d-band centers were obtained asa first moment of n_(d)(E):ε_(d)=∫n_(d)(E)EdE/∫n_(d) (E)dE, wheren_(d)(E) is the projected d-band density of states of the correspondingmaterials. For comparative purpose, pure platinum as a gold standardelectro-catalyst for ORR in fuel cells (PEMFCs, DMFCs) as well as IrO₂widely used for OER in PEM electrolysis were also considered in thepresent study.

For calculating the total energies, electronic structure and density ofelectronic states, the Vienna Ab-initio Simulation Package (VASP) wasused within the projector-augmented wave (PAW) method and thespin-polarized generalized gradient approximation (GGA) for theexchange-correlation energy functional in a form suggested by Perdew andWang. This program calculates the electronic structure and via theHellmann-Feynman theorem, the inter-atomic forces are determined fromfirst-principles. Standard PAW potentials were employed for the Cu, Mn,O, F, Pt and Ir potentials containing eleven, seven, six, seven, ten,and nine valence electrons, respectively. The Cu_(1.5)Mn_(1.5)O₄ at roomtemperature adopts a complex cubic crystal structure with P4₁32 symmetryand space group #213. The bulk elementary unit cell contains 56 atomscorresponding to 8 formula units. All surface calculations for pure andF-doped oxides were done for (100) surface with thirteen atomic layerslab separated by its image in [100] direction by vacuum layer. Both theslab and vacuum layers have the same thickness of ˜12.5 A⁰. In case ofF-doped oxide, 15 F-atoms were randomly distributed over six 8-atomicoxygen layers in the slab, thus representingCu_(1.5)Mn_(1.5)O_(2.75)F_(1.25) composition corresponding to 9.7 wt. %of F. Also, a (111) fcc surface calculation for pure Pt and (110) rutiletype surface calculation for IrO₂ has been conducted for comparativepurposes.

For all the electro-catalysts considered in this study, the plane wavecutoff energy of 520 eV was chosen to maintain a high accuracy of thetotal energy calculations. The lattice parameters and internal positionsof atoms were fully optimized employing the double relaxation procedureand consequently, the minima of the total energies with respect to thelattice parameters and internal ionic positions were determined. Thisgeometry optimization was obtained by minimizing the Hellman-Feynmanforces via a conjugate gradient method, so that the net forces appliedon every ion in the lattice are close to zero. The total electronicenergies were converged within 10⁻⁵ eV/un.cell resulting in the residualforce components on each atom to be lower than 0.01 eV/Å/atom, thusallowing for an accurate determination of the internal structuralparameters for the oxide. The Monkhorst-Pack scheme was used to samplethe Brillouin Zone (BZ) and generate the k-point grid for all thematerials considered in the present patent application. A choice of theappropriate number of k-points in the irreducible part of the BZ wasbased on convergence of the total energy to 0.1 meV/atom.

1. A noble metal-free electro-catalyst composition for use in acidicmedia, comprising: a non-noble metal selected from the group consistingof manganese oxide, titanium oxide, vanadium oxide, iron oxide, chromiumoxide, tantalum oxide, tin oxide, niobium oxide, tungsten oxide,molybdenum oxide, yttrium oxide, scandium oxide, copper oxide, zincoxide, cobalt oxide, nickel oxide, lanthanum oxide, cerium oxide,neodymium oxide, erbium oxide, gadolinium oxide, ytterbium oxide andmixtures thereof, wherein, the noble metal-free electro-catalyst isabsent of any noble metal.
 2. The composition of claim 1, furthercomprising a dopant selected from the group consisting of at least oneelement from Groups III, V, VI and VII of the Periodic Table.
 3. Thecomposition of claim 1, having a general formula I:(a _(x) b _(y))O_(z) :wc  (I) wherein, a is Mn, Cu, Zn, Sc, Fe, Co, Ti,V, Ni, Cr, Ta, Sn, Nb, W, Mo, Y, La, Ce, Nd, Er, Gd, Yb or mixturesthereof; b is Mn; O is oxygen; c is F, Cl, Br, I, S, Se, Te, N, P, As,Sb, Bi, Al, B or mixtures thereof; x and y are each a number greaterthan or equal to 0 and less than or equal to 2, x and y being the sameor different, when x=0, y is a number greater than 0 and less than orequal to 2 and when y=0, x is a number greater than 0 and less than orequal to 2; z is a number that is greater than 0 and less than or equalto 4, and w is from 0% by weight to about 20% by weight, based on totalweight of the composition.
 4. The composition of claim 2, wherein thedopant is selected from the group consisting of fluorine, chlorine,bromine, iodine, sulfur, selenium, tellurium, nitrogen, phosphorus,arsenic, antimony, bismuth, aluminum, boron and mixtures thereof.
 5. Thecomposition of claim 2, wherein the dopant is present in an amount fromgreater than 0 to about 20 weight percent based on total weight of thecomposition.
 6. The composition of claim 2, wherein the dopant ispresent is an amount from about 10 to about 15 weight percent based ontotal weight of the composition.
 7. The composition of claim 1, whereinthe non-noble metal-based electro-catalyst is in the form selected fromthe group consisting of nano-particles, a nanocrystalline thin film,nanorods, nanoshells, nanoflakes, nanotubes, nanoplates, nanospheres,nanowhiskers or combinations thereof.
 8. The composition of claim 3,wherein the non-noble metal is a binary metal oxide comprising copperoxide and manganese oxide, and the dopant is fluorine.
 9. Thecomposition of claim 3, wherein x is 1.5, y is 1.5, z is 4 and w is 0,5, 10 and 15 percent by weight.
 10. The composition of claim 3, whereinx is 1.5, y is 1.5, z is 2.75 and w is 1.25.
 11. The composition ofclaim 1, wherein the acidic media is selected from the group consistingof oxygen reduction reaction in a proton exchange membrane fuel cell,direct methanol fuel cell and oxygen evolution reaction in a protonexchange membrane based water electrolysis.
 12. A method for preparing anoble metal-free electro-catalyst for an acidic electrolyte, comprising:combining a non-noble metal oxide precursor with a precipitation orreaction agent to form a non-noble metal oxide precipitate; separatingthe precipitate; drying the precipitate; and forming a noble metal-freeoxide powder.
 13. The method of claim 12, further comprising introducinga dopant precursor prior to the step of drying the nanoparticles, toform a solid solution.
 14. The method of claim 12, wherein the non-noblemetal oxide precursor is manganese acetate tetrahydrate, theprecipitation agent or reaction agent is potassium permanganate, theprecipitate is manganese dioxide nanoparticles, the dopant precursor isammonium fluoride, and the powder is fluorine-doped manganese oxidepowder.
 15. The method of claim 12, further comprising applying theelectro-catalyst composition to a substrate to form an electrode.